Anticoagulant therapy is effective in the prevention of many thromboembolic disorders, such as stroke, myocardial infarction, and venous thrombembolism. In addition to being the most rapidly growing segment of our population, older patients are also the most likely to have these common indications for anticoagulant therapy. Although anticoagulant therapy has proven benefit in these conditions, it also has an adverse effect--bleeding. Concerns about this adverse effect of anticoagulant therapy may make clinicians less willing to prescribe anticoagulant therapy to older patients who might benefit most from treatment. The main goal of this proposal is to improve the use of anticoagulant therapy in older patients; it will also serve as a model for the investigation of other drug-induced illnesses in older patients Past studies and my own preliminary work indicate that warfarin-related bleeding may be more common in older patients. Which older patients are at greatest risk, and why they are more likely to bleed, is not known. This may lead physicians to erroneously under- or over-estimate the risk of warfarin-related bleeding in older patients, resulting in the withholding of therapy to some older patients whose benefit may outweigh their risk. Additionally, the attitudes and beliefs of physicians and patients about the risks of warfarin-related bleeding may affect the decision to initiate therapy. Clarification of these issues will lead to improvement in the appropriate use of warfarin therapy in older patients. I have designed an incremental research plan with three specific aims. Aims 1 and 2 will address critical unresolved issues involving the safety of anticoagulant therapy in older patients. Aim 3 will extend my research into a new area central to improving the use of anticoagulant therapy in older patients with atrial fibrillation--i.e., patient and physician attitudes concerning the risks and benefits of anticoagulant therapy, and the relationship of their attitudes to the appropriate use of anticoagulant therapy. This research proposal will build directly on my training and current work, while taking advantage of the research environment provided by my sponsors and others at Case Western Reserve University. It will also provide me with the scientific methods and clinical strategies to investigate other drug therapies in older patients, as well as to develop preventive strategies that will lead to improvements in the health care of older persons.